There is now in place a technique for transferring packets sent from source terminals to destination terminals by using a static communication address (terminal identification address), which is pre-assigned to a mobile terminal, and by which the terminal can be identified uniquely, and a dynamic communication address (packet transfer address) assigned to the terminal temporarily. As regarding such a technique, RFC 1631 discloses in “The IP Network Translator (NAT)”, a NAT having a function for, when receiving a packet from a mobile terminal, translating into a packet transfer address a terminal identification address designated in a source address of the packet. As a technique derived from the one disclosed by RFC 1631, RFC 2663 discloses in “The IP Network Address Translator (NAT) Terminology and Considerations”, a network address translator (Twice NAT) having a function for translating, on the basis of an address translation table preset in a cache table, not only a terminal identification address designated in a source address of a packet, but also a terminal identification address designated in a destination address of the packet into corresponding packet transfer addresses.
However, in a case where the above addresses are managed by only the NATs disclosed by RFCs 1631 and 2663, the NATs need to translate addresses each time a packet is sent, which imposes a burden on the NATs.
It therefore becomes necessary to manage addresses without imposing a burden on NATs.
As an example of such an address management method, Japanese Patent Application Laid-Open Publication No. 2001-45050 discloses a technique for managing IP addresses assigned to mobile terminals. In the technique disclosed in the publication, the IP address of IP terminal 1 is managed by using address management server 5, location management server 4, IP packet transfer device 3, and access point 2. Specifically, address management server 5 comprises an address management table for storing IP addresses of terminals for each closed user group (CUG) to which the users of the terminals belong. The address management server 5, on the basis of the table, specifies the CUG associated with the user ID of IP terminal 1 requesting an address, and assigns to IP terminal 1 the IP address for the specified CUG. Location management server 4, on the basis of information sent from access point 2, generates location information of an IP address, which is information indicating access point 2 currently serving IP terminal 1 to which the IP address is assigned. Packet transfer device 3 comprises an IP packet transfer table for storing the CUG associated with the user ID of IP terminal 1, the IP address assigned to IP terminal 1, and location information of the IP address. Packet transfer device 3 transfers packets on the basis of the IP packet transfer table. Access point 2 stores a served user management table for storing information such as user IDs and IP addresses of IP terminals 1 served by access point 2. Access point 2 transfers packets on the basis of the served user management table. When an IP address is no longer required as a result of a communication termination of IP terminal 1, access point 2 notifies IP address management server 5 of the fact. IP address management server 5 then returns the IP address into an unused IP address pool. IP packet transfer device 3 updates its IP packet transfer table, and access point 2 updates its served user management table. Consequently, the IP address is released.
In a case where, as disclosed in RFCs 1631 and 2663, mobile communication is performed by using a terminal identification address of a mobile terminal and a packet transfer address assigned to the terminal, and the addresses are managed as disclosed in the publication. The following communication system is possible by comprising a management device, and relay devices. The management device is provided in a network for managing correspondence (hereinafter, referred to as “address binding information”) between a terminal identification address assigned to a mobile terminal linked to the network, and a packet transfer address. The management server, in response to an inquiry from a relay device, sends to the relay device address binding information. The relay device stores the address binding information. In the communication system, a mobile terminal sends a packet by using terminal identification addresses as a source address and a destination address. A relay device linked to the terminal translates the terminal identification addresses into packet transfer addresses, and sends the resultant packet to a terminal to which the packet is addressed. A relay device linked to the destination terminal, upon receiving the transferred packet, translates packet transfer addresses of the packet into terminal identification addresses, and sends the resultant packet to the destination terminal.
These relay devices store two pieces of information: address binding information of a source terminal of a packet, and address binding information of a destination terminal of the packet. A relay device can assess a memory capacity requirement for storing address binding information of source terminals linked to the relay device. On the other hand, it is difficult for a relay device to assess a memory capacity requirement for storing address binding information of destination terminals of packets, because the number of destination terminals to which packets are addressed is not fixed. Thus, unless a memory capacity of a relay device is increased sufficiently, the device may run out of memory capacity. Also, in a case where a relay device inquires address information from a management device each time the relay device receives packets from mobile terminals, there may occur a problem of congestion on a network because of the movement of large numbers of signals from the inquiries.
In a case where a management device continues to store address binding information of mobile terminals which have terminated communication, there occurs another problem that the relay device runs out of memory capacity for storing any subsequent address binding information. Consequently, there can be conceived a configuration where, when a mobile terminal terminates communication with one communication partner among a plurality of partners, a relay device linked to the terminal requests a management server to delete address binding information of the terminal stored in the server. However, if such a request is sent to the management server each time a mobile terminal terminates communication with a communication partner, a problem of congestion on the network because of the movement of large numbers of signals from the requests would occur.
In a case where a packet transfer address assigned to a mobile terminal is changed, and the change is not reflected in address binding information correctly, the mobile terminal may receive no packets addressed to the terminal.